A 58-year-old man comes to the physician because of a 6-month history of increasing difficulty buttoning his shirt, occasional cramping in his hands, and tripping while walking. He says his symptoms began on the right side but have gradually affected both sides. He denies sensory changes, bladder symptoms, or visual disturbances. On neurologic examination, he has muscle wasting in the hands, hyperreflexia in the lower extremities, and a brisk jaw jerk reflex. Plantar reflexes are upgoing bilaterally, and fasciculations are visible in the tongue. Sensory exam shows no abnormalities. Which of the following is most likely responsible for this patient’s presentation?
Autoimmune demyelination of peripheral motor axons
Degeneration of anterior horn cells and lateral corticospinal tracts
Inhibition of GABAergic interneurons in the spinal cord
Loss of dopaminergic neurons in the substantia nigra pars compacta
Trinucleotide repeat expansion leading to cerebellar degeneration
